The present invention relates to a rotary positive displacement compressor comprising at least one rotor forming compression chambers in a working space, the compressor having an inlet port communicating with a low pressure channel, an outlet port communicating with a high pressure channel, intermediate port means communicating with an intermediate pressure channel and bleed port means selectively connectable to said low pressure channel through a return channel, said intermediate port means and said bleed port means being located such that they face a compression chamber within said working space in a compression chamber, which chamber is sealed from communication with said inlet port as well as said outlet port by said at least one rotor.
The invention further relates to a plant of refrigeration type comprising such a compressor and having a condenser communicating with said high pressure channel, an evaporator communicating with said low pressure channel, a vessel for an intermediate pressure communicating with said intermediate pressure channel, a channel connecting said condenser to said vessel, said channel having first pressure reducing means for decreasing the high pressure in said condenser to the intermediate pressure in said vessel and a channel connecting said vessel to said evaporator, said channel having second pressure reducing means decreasing the intermediate pressure in said vessel to the low pressure in said evaporator.
A compressor and a plant of such types are earlier known from U.S. Pat. No. 3,913,346. The intermediate pressure zone in such plants is used for internal cooling purposes within the plant at a temperature level above that of the evaporator. The main cooling purpose is to precool the liquified refrigerant before the supply thereof to the evaporator which results in a more effective use of the evaporator area so that the dimensions thereof can be minimized for a certain capacity simultaneously as the swept volume of the compressor and thus its dimensions can be reduced correspondingly. Furthermore the power required for recompression of the gaseous refrigerant supplied at the intermediate pressure will be less than that if all the refrigerant were supplied at the evaporator pressure.
In order to vary the volumetric capacity the compressor in U.S. Pat. No. 3,913,346 is provided with a selectively adjustable valve controlling a bleed port in the wall of the working space so that a certain amount of the working fluid supplied to the compressor may be returned to the inlet channel of the compressor. This bleed port is disposed within the same phase of the compression cycle as the intermediate port means. When the bleed port is opened the pressure level inside the compressor working space decreases to such an extent that the back pressure within the area of the intermediate port means will be practically the same as that in the low pressure channel. The bleed port must in order to avoid throttling losses be provided with a large area corresponding to what is required not only for the recirculation of the surplus fluid supplied through the inlet port but also for draining the fluid supplied through the intermediate port means. The size of the valve member will thus be too large for location in the end wall with regard to its area compared with the limited space available outside the rotor bearings. For this reason the valve has to be located in the barrel wall of the working space. Such a valve will consequently be complicated in shape and expensive to manufacture as it not only has to sealingly cooperate with its seat in the housing but also has to sealingly cooperate with the confronting rotor or rotors in order to avoid internal leakage within the compressor, especially when running under maximum capacity conditions.
In the PCT-application with International Publication Number W086/06798 a compressor is disclosed, where the discussed problems related to a compressor and a refrigeration plant of the type in question are overcome by providing a connection controlled by a selectively adjustable over-flow valve between the intermediate pressure channel and the low pressure channel. In this way the need for a separate bleed port is eliminated as the intermediate port means will act as such a port during low volumetric capacity conditions when only the surplus supplied working fluid has to be drained from the working space.
The main object of the present invention is to reach an alternative solution to overcome these problems so as to achieve a more effective capacity control of the compressor per se as well as of a complete refrigeration plant by means of simpler and less expensive valve arrangement than those used in the prior art.
According to one aspect of the invention this object is attained by providing a compressor of the introductionally specified kind with valve means, selectively adjustable between two end positions for formation of different flow paths within the compressor, in the first end position said valve means opens up a direct communication between said intermediate pressure channel and said return channel and opens said bleed port means, whereby fluid flows directly from the intermediate pressure channel to the return channel simultaneously as fluid within the working space flows to the return channel through the intermediate port means as well as through the bleed port means, whereas in the second end position said valve means blocks said direct communication between said intermediate pressure channel and said return channel and closes said bleed port means.
According to another aspect of the invention this object is attained by providing a refrigeration plant of the introductionally specified kind with valve means as specified above.
The main advantage with a compressor and a refrigeration plant according to the invention is the possibilty to optimize the areas of the bleed port means and the intermediate port means, thereby allowing greater freedom for their location and admitting less complicated valve constructions for the bleed port means. The area of the intermediate port means is determined only by what is required for the passage of the intermediate pressure fluid from the intermediate pressure channel to the compressor. At reduced capacity condition when the valve means is in the first end position a part of the partly compressed fluid which is to be recirculated to the inlet flows through the intermediate port means to the return channel. The bleed port means thus can be dimensioned to take care of only the remaining part of the fluid to be recirculated.
Further objects of the invention and how those are met will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings